I think the culture exhibited in the Federation is inherently against genetic manipulation and excessive bio-mechanical technology. They probably have the technology to do it, but few people would chose to have their brains transplanted into an artificial body, even if it was their only chance for survival. It's a step beyond artificial limbs and implants that most simply wouldn't take.
Star Trek will probably always be beholden to its simple 1960s "Wagon Train to the stars" sci-fi roots. It's a setting in which to discuss social issues, do a bit of adventuring, and occasionally have a good gun-fight, but not the best place to run wild with technology. We could easily imagine a sci-fi universe with technology far in advance of Star Trek, but it's no small task trying to merge every new fantastical piece of current theoretical technology into that established universe, and Trek writers have always done it in little baby steps when they have tried. It's usually through a species of the week, not Federation society or Starfleet itself.